RNA editing plays a central role in the life cycle of hepatitis delta virus, a subviral human pathogen. A cellular protein, likely a double- stranded RNA adenosine deaminase, is responsible for editing the antigenomic RNA at the amber/W site. This process changes the function of the viral protein from one of supporting RNA replication to one of facilitating virion formation and inhibiting RNA replication. Editing is highly specific, and requires a particular structure in the HDV RNA. The long-term objectives of the proposal are to expand our understanding of RNA editing via adenosine deamination, and the particular mechanisms through which this process is utilized by HDV. Post-transcriptional regulation by adenosine deamination is increasingly recognized as an important biologic regulatory mechanism that specifically controls the expression of viral and cellular protein variants that have altered functions. The identification and characterization of the RNA structures required, the deaminase enzymes responsible for the modifications, and accessory factors that might influence editing rates and specificity, remain important goals in advancing our understanding of this process. The specific aims are 1) to define the RNA structures required for editing HDV genotype I RNA; 2) to identify the RNA structures required for editing HDV genotype III RNA; 3) to evaluate the effects of modulating RNA adenosine deaminase expression on HDV RNA editing, RNA replication, virion formation, and genetic stability; and 4) to examine the mechanisms and implications of the inhibition of editing by hepatitis delta antigen, the sole viral protein. These aims will be addressed by a combination of approaches, including site- directed mutagenesis, analysis in vitro of editing and RNA-protein interactions, and evaluation of editing and its consequences in transfected cells.